Graydon (oak@uniserve.com) wrote:
<Feathers are integumentary tissue; if (as in the primary feathers of some
modern birds) the feather anchors to bone, that doesn't obviously imply that
it's a rigid anchor, and indeed given that the anchor would have had to evolve
as the aerodynamic function of the feathers developed, it is very hard to see
how it could be a rigid anchor -- the first proto-bird whose tail fan hung them
up on the underbrush when fleeing a predator would tend to be deselected
compared to its compatriots whose tails kept folding..>
The reason I brought up what I did, in case it was unclear, was that in
living birds, the tail feathers are bound in a series of muscles and are not
directly connected to the tail bone (except in the pygostyle itself directly
for the most medial of tail feathers), such that muscular action of a rather
simple nature is all that is required to move the tail feathers, dip them to a
side for yawing, or fanning/narrowing. This system of muscles are present in
*Archaeopteryx*, but only at the base of the tail, where there are VERY few
feathers relative to the entirety of the tail. So while it's possible
*Archaeopteryx* could rearrange easily the base of the tail, the rest should be
less possible without inventing anatomy or adding in material for which we have
no evidence (currently) for.
<Not if the bases of the feathers were connected into ligaments, which is not
observed but would not necessarily be, even in cases of fossilization which
preserve feathers.>
But even so, lack of evidence cannot be used to propose a function. We CAN
argue that Archie had such muscles and ligaments, but it becomes empty
speculation without primary data to support it. Do any other long, bony-tailed
animals possess muscles and ligaments which are used to motivate the integument
in the fashion suggested? I have not seen any, and mammals, which also evolved
their tail musculature from the same amphibian-like root-stock as did dinosaurs
(and specifically here, birds), do not possess nearly the same integument that
would be shape-adaptable save for, say, porcupines or maybe cats and squirrels,
though these muscles are dermal in nature.
<Nor is it likely that original contour feathers from which the aerodynamic
feathers evolved lacked all connection to some mechanism of mobility; even an
emu can fluff its feathers.>
This will not however help the argument for a fanning tail. Emu have the
requisite anatomy that I described in the last post for moving their feathers
in the tail and wing. None of these seem to bear on the issue of whether
*Archaeopteryx* could do this in its tail, which still presumes muscular action
along the stiffened tail to be preserved despite the rigidification of this
structure. Note that the muscles that are adapted in birds were originally
tail-flexing and femoral-swinging muscles, the latter being present only at the
base in *Archaeopteryx* and the former negligible and likely absent given the
absence of flexure ability IN the bulk of the tail.
<But *no* control mechanism, in the face of substantial selection pressure for
flight control, seems extremely far-fetched as an hypothesis.>
It is possible that the muscles and ligaments could have remained to control
the raches there. However, some words of caution on this:
Large aerodynamic feathers appear in animals phylogenetically "older" than
*Archaeopteryx* at the tips of the tail: *Caudipteryx*, *Protarchaeopteryx*,
*Microraptor* (incl. "Cryptovolans"). One would presume a series of
evolutionary forms where the tail became more progressively anteriorly an
aerofoil.
The muscles that birds use (at least) for feather control are proximal (not
distal) and evolved from muscles that were also proximal in the tail, not
distal. In non-avian theropods, the reduction of these muscles, esp. the _m.
caudofemoralis longus_ and _mcf externus_, occured well before the phylogenetic
origin of birds, and thus it is unlikely these muscles would be retained in the
stock that gave rise to *Archaeopteryx*, requiring only that the tail become
shorter for the feathers to become embedded in the requisite musculature.
It is possible that the reduction of the tail caused loss of the distal
caudals directly and at the same time loss of the distal/medial feathers,
meaning that avian tail feathers evolved from only the proximal tail feathers
and the first 10 or so caudal vertebrae seen in brutes like *Allosaurus*,
rather than the distal tip. The pygostyle developed, likely, from pressures to
control the distal tail's feathers, and would simply have redeveloped no matter
how many caudals were present as the genes remained active even if the caudal
count decreased.
<Individual wing feathers can be moved by extant birds. I can't tell you what
the mechanism is, but watching a large soaring bird fly makes it quite obvious
that individual feather position adjustment is taking place.>
These would be only the distal most, hand-based feathers in the wing, not the
proximal ones, which remain connected to individual finger bones and can
involve no minor a moment than a twitch of a "knuckle" to move so slightly. The
alula is connected to the first digit's primary phalanx, for example.
In summary:
It is thus that I do not think *Archaeopteryx* possessed much of a means to
fan the tail, but that it, as Jim Cunningham suggests, might act to reduce drag
(which works better in a running animal than in a flying animal, anyway). To
put this in contect, this argument would aid WAIR models and the model Scott
Hartmann proposed at SVP a few years back, whereby running, chasing and such
behavior forced evolution of distal aerofoils, preceding the states where these
foils became more proximally located and WAIR behavior preceded gliding and
then flight behavior. This is also a resurrection of Ostrom's theories
regarding the origin of birds, but without the "bird net" of the arm wings.
This argument does not represent my personal belief, of which I have none.
Cheers,
Jaime A. Headden
http://bitestuff.blogspot.com/
"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
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